The present invention relates to a reduction starter having a planetary gear type reduction mechanism disposed between a starter motor and a pinion and now particularly, to a planetary gear type reduction starter for internal combustion engines.
Recently, there has been a demand to provide automotive vehicles with driving systems of lightweight and compact design, such as F.F. (front engine--front wheel drive) type driving system, to improve the fuel consumption rate and the riding comfort of the vehicles, with such demand also being raised with regard to engine starters. In view of these demands, a planetary gear type reduction starter have been proposed in U.K. Patent Specification No. 964,675 as a substitution for the conventional engine starter having a parallel shaft type reduction gear mechanism in which the axis of the motor shaft is parallel to and spaced from the axis of the output pinion shaft. With the planetary gear type reduction mechanism, it is possible to arrange the pinion shaft coaxially with the motor shaft, so that the size of the reduction mechanism can be considerably reduced.
In the planetary gear type reduction starter, a plurality of planet gears are disposed in an annular space between a sun gear and an outer ring gear at circumferentially equal intervals. If the load cannot be distributed equally or uniformly to all of the planet gears, an unduly increased load is applied to only some of the planet gears thereby disadvantageously decreasing the load capacity or performance of the planetary gear reduction mechanism and increasing in vibration and noise levels. In order to avoid such problems, the gears of the planetary gear type reduction mechanism must be fabricated and assembled with a high precision which, however, increases the manufacturing cost.
In order to attain a uniform distribution of load to all of the planet gears, it has been attempted to fabricate more than one of the gears of a planetary gear mechanism from a resilient or flexible material. It has also been attempted to resiliently support either one of the gears of a planetary gear mechanism.
As the former attempt, an internally toothed outer ring gear of the planetary gear mechanism has been designed to have a decreased radial thickness of the rim section of the ring gear. However, due to the concerns regarding the reduction of the mechanical strength of the ring gear, the rim thickness of the ring gear was not decreased sufficiently to attain a good distribution of load to all of the planet gears.
It is an object of the present invention to provide a planetary gear type reduction starter with an improved load performance of the planetary gear mechanism.
It is another object of the present invention to provide a starter with reduced vibration and noise levels.
It is a further object of the present invention to provide a starter which can be manufactured at a reduced cost.
The reduction starter according to the present invention comprises a starter motor; a planetary gear reduction mechanism including a sun gear driven by the starter motor, an internally toothed outer ring gear locked against rotation and a plurality of planet gears disposed in meshing engagement with the sun gear and the outer ring gear and mounted for revolution about the axis of the sun gear; and means for transmitting the revolution of the planet gears to a crank shaft of an associated internal combustion engine. The outer ring gear includes an outer rim section having a radial thickness t and a plurality of radially inwardly extending gear teeth each having a height h. The rim thickness t is determined to fall within the range of from EQU t.ltoreq.0.8h
to EQU t.gtoreq.0.4h.
As will be seen from the above, the thickness of the rim section of the outer ring gear is less than the radial height of each of the radially inwardly extending gear teeth of the ring gear. Thus, the rim section is flexible or elastically deformable to improve the uniformity of the distribution of the load torque to all of the planet gears, which improves the load performance of the planetary gear type reduction mechanism. Due to the deformable design of the outer ring gear, the stress produced by the load torque in the root of each gear tooth is substantially equalized to the bending stress produced in the outer periphery of the ring gear at a point substantially radially outward of the loaded gear tooth, whereby the allowable load torque of the ring gear can be increased to contribute to the reduction in the size and weight of the starter. The improved load distribution assures a well balanced operation of the planetary gear reduction mechanism with resultant improvement in the efficiency of the starter operation and reduction in the noise and vibration produced. Moreover, the flexibility of the rim of the outer sun gear is opreative to absorb offsets of component parts of the planetary gear reduction mechanism from correct positions caused due to less precise mounting. In other words, the deformable nature of the outer ring gear is effective to render the planetary gear mechanism insensitive to precision of fabrication of the gears and of mounting thereof to thereby assure reduction in the cost of manufacture of the planetary gear reduction mechanism and thus of the reduction starter.
The above and other objects, features and advantages of the present invention will be become more apparent from the following description when taken in connection with the accompanying drawings.